Systems and methods for measuring isolation resistance

ABSTRACT

An energy storage system may include battery packs, such that one terminal of the battery packs is electrically coupled to a resistor representative of an isolation resistance of the battery system. The system may include a semiconductor relay switch, a plurality of resistors configured to electrically couple to the battery packs via the semiconductor relay switch, a gain field-effect transistor (FET) configured to electrically short at least one resistor of the plurality of resistors, and a control system. The control system may send a first command to the semiconductor switch to close, acquire a first voltage waveform, send a second command to the semiconductor switch to open, send a third command to the gain FET to close, send a fourth command to the semiconductor switch to close, acquire a second voltage waveform, and determine the isolation resistance based on the first voltage waveform and the second voltage waveform.

BACKGROUND

The present disclosure generally relates to the field of batteries andbattery systems. More specifically, the present disclosure relates toisolation barrier fault detection for a battery system.

This section is intended to introduce the reader to various aspects ofart that may be related to various aspects of the present disclosure,which are described below. This discussion is believed to be helpful inproviding the reader with background information to facilitate a betterunderstanding of the various aspects of the present disclosure.Accordingly, it should be understood that these statements are to beread in this light, and not as admissions of prior art.

Electrical systems often include a battery system to capture (e.g.,store) generated electrical energy and/or to supply electrical power.For example, a stationary power system may include a battery system thatreceives electrical power output by an electrical generator and storesthe electrical power as electrical energy. In this manner, the batterysystem may supply electrical power to electrical components using thestored electrical energy.

Additionally, a battery system may be included in the electrical systemof an automotive vehicle to supply electrical power used to supplementthe motive force (e.g., power) of the automotive vehicle. Such anautomotive vehicle may be referred to as an xEV, where the term “xEV” isdefined herein to include all of the following vehicles, or anyvariations or combinations thereof, that use electrical power tosupplement vehicular motive force. For example, electric vehicles (EVs)may include a battery system that supplies electrical power to abattery-powered electric propulsion system (e.g., one or more motors),which provides all vehicular motive force. Additionally, hybrid electricvehicles (HEVs), also considered xEVs, may provide vehicular motiveforce using a combination of an internal combustion engine propulsionsystem and a battery-powered electric propulsion system, for example,supplied by a 48 volt or a 130 volt battery system.

In any case, electrical components in an electrical system may operateusing differing voltage domains (e.g., ranges). For example, an electricmotor may operate using high voltage (e.g., 48 volt) electrical power,whereas a control system may operate using low voltage (e.g., 12 volt)electrical power. To facilitate implementing multiple different voltagedomains, one or more isolation barriers may be used between differentvoltage domains, for example, between electrical components and/orbetween electrical components and a common (e.g., system) ground. Insome instances, faults in an isolation barrier may affect operation ofthe electrical system and/or the battery system. With this in mind, itis now recognized that improved systems and techniques for monitoringthe isolation properties (e.g., resistance) of an isolation barrier mayimprove fault detection efficiency and, thus, operation of theelectrical system and/or the battery system.

SUMMARY

A summary of certain embodiments disclosed herein is set forth below. Itshould be understood that these aspects are presented merely to providethe reader with a brief summary of these certain embodiments and thatthese aspects are not intended to limit the scope of this disclosure.Indeed, this disclosure may encompass a variety of aspects that may notbe set forth below.

To facilitate implementing multiple different voltage domains, in someembodiments, the battery system may include distinct sets of batterymodules (e.g., packs) that output voltages in the different voltagedomains. Additionally or alternatively, the electrical system mayinclude a converter to convert electrical power between the differentvoltage domains. Furthermore, in some embodiments, the battery systemmay provide electrical power in a first voltage domain and an externalpower source (e.g., grid) may provide electrical power in a secondvoltage domain.

Moreover, one or more isolation barriers may be included in the batterysystem to reduce likelihood of electrical power deviating from target(e.g., desired) paths. For example, an undesired connection (e.g.,short) between different voltage domains may affect voltage supplied toand, thus, operation of electrical components in the electricalsystem—particularly when the voltage deviates from target operatingvoltage of the electrical components. As such, an isolation barrier maybe included between different voltage domains in the electrical system.For example, an isolation barrier may be implemented between electricalcomponents operating in a first (e.g., high) voltage domain andelectrical components operating in a second (e.g., low) voltage domain.Additionally or alternatively, an isolation barrier may be implementedbetween electrical components and a common ground (e.g., vehicle chassisand/or housing of the electrical system). In either case, one or moreisolation barriers may be employed to reduce likelihood of faultsaffecting the operation of the battery system, operation of theelectrical system, and/or the surrounding environment.

Accordingly, the present disclosure provides techniques to improveoperation of a battery system and/or an electrical system by improvingdetection efficiency of faults occurring in one or more isolationbarriers. In some embodiments, the battery system may include anisolation measurement circuit that regularly determines whether a faultis present in the one or more isolation barriers. In particular, theisolation measurement circuit may facilitate determining properties(e.g., isolation resistance) of an isolation barrier indicative ofwhether a fault in the isolation barrier is expected to be present. Forexample, the isolation measurement circuit may output measurements usedto determine isolation resistance between electrical componentsseparated by an isolation barrier and/or isolation resistance betweenelectrical components and a common ground separated by an isolationbarrier.

DRAWINGS

Various aspects of this disclosure may be better understood upon readingthe following detailed description and upon reference to the drawings inwhich:

FIG. 1 is a block diagram of an electrical system, in accordance with anembodiment presented herein;

FIG. 2 is a cutaway schematic view of a vehicle including the electricalsystem of FIG. 1 implemented using a battery system, in accordance withan embodiment presented herein;

FIG. 3 is a block diagram of the battery system of FIG. 2 including anisolation measurement circuit, in accordance with an embodimentpresented herein;

FIG. 4 is a schematic diagram of the isolation measurement circuit ofFIG. 3, in accordance with an embodiment presented herein;

FIG. 5 is a flow chart of a process for operating the isolationmeasurement circuit of FIG. 3, in accordance with an embodimentpresented herein;

FIG. 6 is a plot of example waveforms determined by the isolationmeasurement circuit of FIG. 3, in accordance with an embodimentpresented herein; and

FIG. 7 is a flow chart of a process for determining isolation propertiesof an isolation barrier using the isolation measurement circuit of FIG.4, in accordance with an embodiment presented herein.

DETAILED DESCRIPTION

One or more specific embodiments will be described below. In an effortto provide a concise description of these embodiments, not all featuresof an actual implementation are described in the specification. Itshould be appreciated that in the development of any such actualimplementation, as in any engineering or design project, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which may vary from one implementation toanother. Moreover, it should be appreciated that such a developmenteffort might be complex and time consuming, but would nevertheless be aroutine undertaking of design, fabrication, and manufacture for those ofordinary skill having the benefit of this disclosure.

The present disclosure relates to batteries and battery systems. Morespecifically, the present disclosure relates to determining isolationproperties of an isolation barrier between different voltage domains ina battery system and/or an electrical system utilizing the batterysystem.

Generally, an electrical system may include a battery system to capture(e.g., store) generated electrical energy and/or to supply electricalpower to electrical components (e.g., equipment, machines, and/ordevices). For example, an automotive vehicle may include a batterysystem to supply electrical power to an electric motor, a batterycontrol unit, a vehicle control unit, radio, and/or lights. In someembodiments, different electrical components may operate usingelectrical power in different voltage domains. For example, the electricmotor may operate using operate using high voltage (e.g., 48 volt)electrical power, whereas the vehicle control unit may operate using lowvoltage (e.g., 12 volt) electrical power.

Accordingly, as mentioned above, the present disclosure providestechniques to improve operation of a battery system and/or an electricalsystem by improving detection efficiency of faults occurring in one ormore isolation barriers. In some embodiments, the isolation measurementcircuit may facilitate determining the isolation resistance withoutdirectly coupling the isolation measurement circuit or the one or morebattery modules to the common ground. To facilitate determining theisolation resistance, the isolation measurement circuit may be coupledto a voltage source (e.g., one of the battery modules in the batterysystem). Additionally, in some embodiments, the isolation measurementcircuit may also include a semiconductor relay switch (e.g., aPhotoMOS®), a number of gain field effect transistors (FETs), and/or anumber of selectively connectable resistors.

In operation, the isolation measurement circuit may close thesemiconductor relay switch and couple a first resistor circuit having afirst known resistance to the battery module, for example, bymaintaining a first gain FET open. When the semiconductor relay switchis closed, the isolation measurement circuit may capture (e.g., output)a first voltage waveform indicative of the isolation resistance of anisolation barrier between the voltage source and the common ground. Forexample, the first voltage waveform may indicate a firstresistor-capacitor decay due to capacitance and resistance (e.g., thefirst known resistance and the isolation resistance) between the voltagesource and the common ground. Based at least in part on the firstvoltage waveform and the first known resistance, in some embodiments, acontrol system (e.g., battery control unit) may determine the isolationresistance between the voltage source and the common ground.

However, in some instances, the capacitance between the voltage sourceand the common ground may vary (e.g., difficult to accuratelydetermine), thereby affecting accuracy of the isolation resistancedetermined using only the first voltage waveform. To improve accuracy,in some embodiments, the isolation measurement circuit may close thesemiconductor relay switch and couple a second resistor circuit having asecond known resistance to the battery module, for example by closingthe first gain FET. When the semiconductor relay switch is closed, theisolation measurement circuit may capture (e.g., output) a secondvoltage waveform that, for example, indicates a secondresistor-capacitor decay due to the capacitance and resistance (e.g.,the second known resistance and the isolation resistance) between thevoltage source and the common ground.

Based at least on the two voltage waveforms and the two knownresistances, the control system may determine the isolation resistancebetween the voltage source and the common ground. By utilizing the twovoltage waveforms, the control system may determine the isolationresistance agnostic of the capacitance between the voltage source andthe common ground. In this manner, the likelihood of variations in thecapacitance affecting determination of the isolation resistance may bereduced.

Additionally, based at least in part on the isolation resistance, thecontrol system may determine when a fault is expected to be present inthe isolation barrier and control operation of the battery system and/orthe electrical system accordingly. For example, the control system maydetermine that a fault is expected to be present when the isolationresistance is below a threshold resistance (e.g., minimum resistanceexpected to sufficient isolate two voltage domains). Additionally, insome embodiments, the control system may send a control commandinstructing a relay or circuit breaker to disconnect the voltage sourcewhen a fault in the isolation barrier is expected to be present. In thismanner, likelihood of a fault in the isolation barrier affecting thebattery system, the electrical system, and/or the surroundingenvironment may be reduced.

To help illustrate, a block diagram of an electrical system 10 is shownin FIG. 1. In the depicted embodiment, the electrical system 10 includesan isolation measurement circuit 12, an isolation barrier 14, a commonground 16, a one or more first electrical components 18, a second one ormore electrical components 20, a first voltage source 22, and a secondvoltage source 24. In some embodiments, the first electrical components18 and/or the second electrical components 20 may include anycombination of equipment, machines, and devices that operate at least inpart using electrical power. Additionally, in some embodiments, thefirst electrical components 18 may be designed (e.g., targeted) tooperate using electrical power in a first voltage domain supplied by thefirst voltage source 22 and the second electrical components 20 may bedesigned to operate using electrical power in a second voltage domainsupplied by the second voltage source 24.

In some embodiments, the target operating voltage of the firstelectrical components 18 and the second electrical components 20 may bedifferent. For example, the target operating voltage of the firstelectrical components 18 may be in a 48 volt domain (e.g., range ofvoltages around 48 volts). On the other hand, the target operatingvoltage of the second electrical components 20 may be in a 12 voltdomain (e.g., range of voltages around 12 volts). In other words, thefirst electrical components 18 may operate in a high voltage domain andthe second electrical components 20 may operate in a low voltage domain,or vice versa.

As described above, the isolation barrier 14 may be implemented toelectrically isolate different voltage domains. Thus, in the depictedembodiment, the isolation barrier 14 may be implemented to electricallyisolate the first electrical components 18 from the second electricalcomponents 20. Furthermore, since voltage domain (e.g., zero volts) ofthe common ground 16 may vary from the target operating voltage of thefirst electrical components 18 and/or the target operating voltage ofthe second electrical components 20, the isolation barrier 14 mayadditionally or alternatively electrical isolate the common ground 16from the first electrical components 18 and/or the second electricalcomponents 20.

To reduce likelihood of a fault in the isolation barrier affectingoperation of the electrical system 10 and/or the surroundingenvironment, the isolation measurement circuit 12 may facilitatedetermining properties (e.g., isolation resistance) indicative of afault in the isolation barrier 14. For example, the isolationmeasurement circuit 12 may facilitate determining isolation resistancebetween the first electrical component 18 and the second electricalcomponent 20. Additionally or alternatively, the isolation measurementcircuit 12 may facilitate determining isolation resistance between thecommon ground 16 and the first electrical components 18 and/or isolationresistance between the common ground 16 and the second electricalcomponents 20.

Furthermore, in some embodiments, the isolation measurement circuit 12may facilitate determining voltage and current measurements in theelectrical system 10. For example, the isolation measurement circuit 12may facilitate determining voltage of the first voltage source 22,voltage of the second voltage source 24, current supplied to the firstelectrical component 18, and/or current supplied to the secondelectrical components 20. In other words, the isolation measurementcircuit 12 may be implemented to facilitate determining otheroperational parameters of the electrical system 10 in addition toisolation resistance—particularly when the isolation measurement circuit12 is electrically coupled to a high voltage domain.

By way of a non-limiting example, the electrical system 10 may beimplemented in an automotive vehicle using battery system 26 as shown inFIG. 2. As depicted, the battery system 26 includes an energy storagecomponent 27 electrically coupled to various electrical components inthe automotive vehicle, such as an ignition system, an alternator, avehicle console, an electric motor, and the like. As described above, insome embodiments, different electrical components in the electricalsystem 10 may operate using electrical power in different voltagedomains. For example, an electric motor may operate using electricalpower in a 48 volt domain and a battery control unit 36 (e.g., controlsystem) may operate using electrical power in a 12 volt domain.

Thus, in some embodiments, the battery system 26 may include multiplebattery modules (e.g., packs) arranged to supply electrical power indifferent voltage domains. For example, in the depicted embodiments, theenergy storage component 27 includes a first battery module 28 (e.g.,first voltage source 22) that supplies electrical power in a firstvoltage domain and a second battery module 30 (e.g., second voltagesource 30) that supply electrical power in a second voltage domain.Although depicted adjacent to one another, in some embodiments, thefirst battery module 28 and the second battery module 30 may bepositioned in areas of the vehicle. Furthermore, in other embodiments,the energy storage component 27 may include any number of batterymodules.

To facilitate receiving and/or supplying electrical power, each batterymodule may include a first terminal 32 and a second terminal 34. In someembodiments, the first terminal 32 may provide a positive voltageconnection and the second terminal 34 may provide a battery systemground connection, which may be separated from the common ground 16 byone or more capacitors. As such, voltage at the battery system groundand the common ground 16 may be different.

A more detailed view of an embodiment of the energy storage component 27is shown in FIG. 3. As depicted, the energy storage component 27includes the first battery module 28, the second battery module 30, thebattery control unit 36, and the isolation measurement circuit 12. Insome embodiments, the battery control unit 36 may generally controloperation of the battery system 26, for example, by monitoringoperational parameters related to the first battery module 28 and thesecond battery module 30. Moreover, the battery control unit 36 maycontrol operation of the isolation measurement circuit 12, one or morerelays electrically connected to the energy storage component 27, andthe like.

To facilitate controlling various operations, the battery control unit36 may include memory 38 and a processor 40. In some embodiments, theprocessor 40 may execute instructions stored in the memory 38. Thus, insome embodiments, the processor 40 may include may include one or moregeneral-purpose microprocessors, one or more application specificprocessors (ASICs), one or more field programmable logic arrays (FPGAs),or any combination thereof. Additionally, in some embodiments, thememory 38 may include one or more tangible, non-transitory,computer-readable mediums. For example, the memory 38 may include randomaccess memory (RAM), read only memory (ROM), rewritable non-volatilememory such as flash memory, hard drives, optical discs, and the like.Furthermore, in some embodiments, the battery control unit 36 may beincluded portions in a vehicle control unit (VCU) and/or as a separatecontrol unit.

As described above, faults in the isolation barrier 14 may be detectedbased at least in part on isolation resistance across the isolationbarrier 14. To facilitate detecting occurrence of a fault, the isolationmeasurement circuit 12 may facilitate determining isolation resistancebetween the first battery module 28 and the second battery module 30.Additionally or alternative, the isolation measurement circuit 12 mayfacilitate determining isolation resistance between the common ground 16and the first battery module 28 or the second battery module 30. Thus,as depicted, the isolation measurement circuit 12 is electricallycoupled to the first battery module 28 and the second battery module 30.

With the foregoing in mind, FIG. 4 illustrates an example implementationof the isolation measurement circuit 12. Although FIG. 4 illustrates anumber of components as being part of the isolation measurement circuit12, it should be understood that the depicted components are exemplarycomponents, and the isolation measurement circuit 12 may employ avariety of different suitable components instead.

Referring to FIG. 4, the isolation measurement circuit 12 may beselectively coupled to a voltage source 66, which supplies electricalpower in a specific voltage domain. Thus, in some embodiments, thevoltage source 66 may include the first battery module 28 and/or thesecond battery module 30. Additionally, in certain embodiments, thevoltage source 66 may correspond to the battery module that supplieselectrical power in the higher voltage domain. As such, the componentsof the isolation measurement circuit 12 may already be exposed to thehigh voltage domain, thereby obviating additional hardware or protectioncomponents to isolate the components of the isolation measurementcircuit 12 from the higher voltage domain. In this manner, the isolationmeasurement circuit 12 may be implemented to facilitate determiningoperational parameters including isolation resistance generallydetermined in the high voltage domain.

In the depicted embodiment, the isolation measurement circuit 12 may beselectively coupled to a voltage source 66 via a semiconductor relayswitch 72 (e.g., PhotoMOS®). In some embodiments, the semiconductorrelay switch 72 may use a light-emitting diode (LED) as an input and ametal-oxide-semiconductor field-effect transistor (MOSFET) as an output.Thus, by implementing the semiconductor relay switch 72, the voltagesource 66 may be selectively connected to the isolation measurementcircuit 12 while remaining electrically isolated from the common ground16. In this manner, likelihood of the voltage source 66 affecting theenvironmental surrounding when determining isolation resistance may bereduced. Generally, PhotoMOS-type semiconductor relay switches areisolated from the circuit that they are switching between and have lowleakage properties. Since isolation between circuits assist in theprocess in measuring the insulation resistance, the isolation featuresprovided by the PhotoMOS-type semiconductor relay switches better enablethe isolation measurement circuit 12 to measure the isolationresistance.

In particular, the semiconductor relay switch 72 may selectively connectone or more resistors with known resistances to the voltage source 66.For example, in the depicted embodiment, the semiconductor relay switch72 may selectively connect some combination of a first resistor 74, asecond resistor 76, a third resistor 78, a fourth resistor 80, and afifth resistor 82 to the voltage source 66. In some embodiments, theresistors 74-82 may be sized in a variety of suitable resistances basedon the properties of the battery system 26. It should be appreciatedthat although FIG. 4 depicts five distinct resistances, in otherembodiments, the isolation measurement circuit 12 may include any numberof resistors. For example, any of the resistors 74-82 may also beimplemented as a single resistor, a series of individual resistors,parallel resistors, or a combination of series and parallel resistors.Additionally, multiple of the resistors 74-82 may be combined.

As depicted, the isolation measurement circuit 12 also includes a firstgain field-effect transistor (FET) 84 and a second gain FET 86. In someembodiments, the first gain FET 84 and/or the second gain FET 86 may beopened and closed to adjust resistance connected to the voltage source66 via the isolation measurement circuit 12. In particular, the firstgain FET 84 may be opened and closed to control when the third resistor78 is connected in the isolation measurement circuit 12. Additionally,the second gain FET 86 may be opened and closed to control when thefourth resistor 80 is connected in the is connected in the isolationmeasurement circuit 12. In one embodiment, the gain FET 84 may be a 200volt amplifier device and the gain FET 86 is a lower voltage (e.g.,10-20V) amplifier device. In addition, the gain FET 84 and the gain FET86 may each include two or three switches that may be controlled viacontrol commands (e.g., gate signals) received from a control system(e.g., the battery control unit 36). In one embodiment, the batterycontrol unit 36 may control the operation of each switch and each gainFET of the isolation measurement circuit 12, but it should be noted thatany suitable processor or processing device may do the same.

In operation, the battery control unit 36 may determine an isolationresistance 88 between a positive bus (e.g., positive terminal of thevoltage source 66) and the common ground 16. Additionally oralternatively, the battery control unit 36 may determine an isolationresistance 90 between a negative bus (e.g., battery system ground) andthe common ground 16. That is, the battery control unit 36 may firstclose the semiconductor relay switch 72 and acquire a first RC decaywaveform. The battery control unit 36 may then open the semiconductorrelay switch 72 and close the gain FET 84, thereby effectively removingthe resistor 78 from the isolation measurement circuit 12. The batterycontrol unit 36 may then close the semiconductor relay switch 72 againand acquire a second RC decay waveform. Using the two acquired RC decaywaveforms and the known resistances of the isolation measurement circuit12, the battery control unit 36 may determine the isolation resistance88 and/or the isolation resistance 90, for example, depending whetherthe first switch 62 or the second switch 64 is closed. It should benoted that the resistances 88 and 90 and switches 62 and 64 are not partof the isolation measurement circuit 12. Instead, these components areprovided to for simulation of the circuit design and are not part of theembodiment that is used to measure the isolation resistance, asdiscussed above.

With the foregoing in mind, FIG. 5 describes a process 100 for operatingthe isolation measurement circuit 12 to facilitate determining theisolation resistance 88 and/or the isolation resistance 90. In oneembodiment, the process 100 may be performed by the battery control unit36 (e.g., via the processor 40), but it should be noted that anysuitable processor may perform the process 100. Although the process 100is described as being performed in a particular order, it should beunderstood that the process 100 may be performed in any suitable order.

Referring now to FIG. 5, at block 102, the battery control unit 36 maysend a control command (e.g., gate signal) instructing the semiconductorrelay switch 72 to close. In some embodiments, prior to sending thiscontrol command, the battery control unit 36 may confirm that the firstgain FET 84 is open. After the semiconductor relay switch 72 closes, atblock 104, the battery control unit 36 may acquire a first voltagewaveform via the output voltage node (V_(out)) of the isolationmeasurement circuit 12. In some embodiments, the first voltage waveformmay correspond to a first RC decay, which may be represented by Equation1 below.

$\begin{matrix}{{Vout} = {V\; 1\left( {1 - e^{- \frac{t}{RC}}} \right)}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where V_(out) corresponds to the output voltage node of the isolationmeasurement circuit 12, V1 corresponds to the voltage source 66, Rcorresponds to the equivalent resistance between the voltage source 66and the common ground 16, and C corresponds to the capacitance (e.g., ofcapacitor 68 and/or capacitor 70) between the voltage source 66 and thecommon ground 16. In some embodiments, the V_(out) node may be coupledto an analog-to-digital converter (ADC) that may be used to determinethe first voltage waveform for processing by the battery control unit36.

For illustration purposes, an example the first voltage waveform thatmay be determined is in graph 120 of FIG. 6. Referring briefly to FIG.6, the first RC decay waveform measurement is represented by curve 122.

Based at least in part on the first voltage waveform, the batterycontrol unit 36 may obtain four voltage samples at four distinct timesduring the decay portion of the curve, such that adjacent times of thefour times have an equal amount of time between each other. The fourvoltage samples may then be used to determine the derivative of Equation1, as shown in Equation 2 below.

$\begin{matrix}{\frac{dv}{dt} = {\frac{1}{RC}*e^{- \frac{t}{RC}}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

Equation 2 operates on a premise of that the time derivative values(e.g., dt) are equal in a given sample window. It should be noted thatthe time derivative values may correspond to the first time constantarea of the curve 122 and are approximately between 30-70 ms. Additionaldetails with regard to how the time derivative values are used todetermine the isolation resistance of the isolation measurement circuit12 will be discussed below with reference to FIG. 7. In someembodiments, to facilitate achieving the target sampling rate,determination of the samples may be implemented via hardware circuitry.

Referring back to FIG. 5, at block 106, the battery control unit 36 maysend a control command to the semiconductor relay switch 72 to openafter the first voltage waveform is determined. The battery control unit36 may then, at block 108, send a control command instructing the firstgain FET 84 to close. After the first gain FET 84 closes, at block 110,the battery control unit 36 may send another control command instructingthe semiconductor relay switch 72 to close while the gain FET switch 84is closed.

At block 112, the battery control unit 36 may acquire a second voltagewaveform when the first gain FET switch 84 is closed and resistor 78 iseffectively disconnected from the isolation measurement circuit 12 and,thus, the voltage source 66. Based at least in part on the secondvoltage waveform, the battery control unit 36 may again obtain fourvoltage samples at four distinct times during the decay portion of thecurve, such that adjacent times of the four times have an equal amountof time between each other. An example second voltage waveform isdepicted with curve 124 of FIG. 6. As shown in FIG. 6, the slope of thecurve 124 is different from the slope of the curve 122. The differencebetween these two curves is attributed to the difference between theresistance of the isolation measurement circuit 12 connects to thevoltage source 66.

After acquiring the second voltage waveform, at block 114, the batterycontrol unit 36 may determine a Thevenin equivalent resistance of theisolation resistance 88 and/or the isolation resistance 90 based atleast in part on the first voltage waveform and the second voltagewaveform. In some embodiments, the battery control unit 36 may regularlymonitor the isolation resistance 88 and compare the isolation resistanceto a threshold resistance. If the isolation resistance 88 is below thethreshold, the battery control unit 36 may send a control commandinstructing to a relay or circuit breaker to disconnect the voltagesource 66 from the battery system 26 and/or the electrical system 10. Assuch, the battery control unit 36 may effectively remove a power sourcewhen a fault in the isolation barrier 14 is expected to have occurred,thereby reducing likelihood of the fault affecting the battery system26, the electrical system 12, and/or the surrounding environment.

By utilizing the first voltage waveform and the second voltage waveform,the battery control unit 36 may determine the isolation resistance withthe assumption that the capacitance between the voltage source 66 andthe common control 16 is relatively constant when determined. In thismanner, the battery control unit 36 may reduce likelihood of uncertaintyin the capacitance of the capacitors 68 and/or 70 on determination ofthe isolation resistance, thereby improving accuracy of the isolationresistance determination. However, in some instances, the capacitancebetween the voltage source 66 and the common ground 16 may be determinedwith relative accuracy, for example, upon startup while the voltagesource 66. In such instances, the battery control unit 36 may use aknown capacitance to determine the isolation resistance using only onevoltage waveform.

FIG. 7 describes a process 130 for determining the isolation resistancebased at least in part on the determined one or more voltage waveforms.As discussed above with reference to the process 100 of FIG. 6, thefollowing description of the process 130 is described as being performedby the battery control unit 36 but it should be understood that anysuitable processor may perform the process 130. Moreover, although thefollowing description of the process 130 is described in a particularorder, it should be noted that the method 130 may be performed in anysuitable order.

Referring now to FIG. 7, at block 132, the battery control unit 36 mayreceive at least four voltage measurements from the a RC decay waveformmeasurement, for example, determined at block 104 of the process 100. Insome embodiments, the voltage measurements may be acquired during thedecay portion of the first RC decay waveform. As discussed above, thefirst RC decay waveform measurement is acquired when the first gain FET84 is open. The four voltage values may be denoted as voltage V1 at timet1, voltage V2 at time t2, voltage V3 at time t2, and voltage V4 at timet4 with respect to the first RC decay waveform measurement (e.g., curve122).

Using these voltage measurements, at block 134, the battery control unit36 may determine a first time constant τ₁ based on the four voltagemeasurements. For instance, based on the four voltage measurements, thebattery control unit 36 may obtain two differential voltages (DV):

DV1=V2−V1 and  (1)

DV2=V4−V3.  (2)

As mentioned above, the time difference between each adjacent voltagemeasurement may be substantially equal. As such, the difference betweentime t2 and time t1 should be equal to the difference between time t4and time t3, as provided below in Equation 3.

t2−t1=t4−t3  Equation 3

Based on the time relationship described in Equation 3, the batterycontrol unit 36 may obtain two voltage differential values (e.g.,dv/dt). Accordingly, based on the time relationship of Equation 3 andthe acquired voltage waveform measurement characteristic curve ofEquation 1, the two voltage differential values may be describedaccording to Equations 4 and 5 below.

$\begin{matrix}{\frac{{dv}\; 1}{{dt}\; 1} = {\frac{1}{R\; 1C}*e^{- \frac{t\; 2}{R\; 1C}}}} & {{Equation}\mspace{14mu} 4} \\{\frac{{dv}\; 2}{{dt}\; 2} = {\frac{1}{R\; 1C}*e^{- \frac{t\; 4}{R\; 1C}}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

where R1 is a determined isolation resistance based on the firstacquired RC decay waveform measurement.

By rearranging Equations 4 and 5 to solve for the time differential dt,the battery control unit 36 may set Equation 4 equal to Equation 5 sincedt1=dt2. As a result, the battery control unit 36 may obtain Equation 6that represents the first time constant τ₁.

$\begin{matrix}{\tau_{1} = \frac{{t\; 4} - {t\; 2}}{\ln \left( \frac{{dv}\; 1}{{dv}\; 2} \right)}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

Referring back to the process 130 of FIG. 6, at block 136, the batterycontrol unit 36 may receive four voltage measurements (e.g., voltage V5at time t5, voltage V6 at time t6, voltage V7 at time t7, and voltage V8at time t8) from the second RC decay waveform measurement, for example,determined at block 112 of the process 100. Like the first four voltagemeasurements received at block 132, the time difference between eachadjacent voltage measurement may be substantially equal, as providedbelow in Equation 7.

t6−t5=t8−t7  Equation 7

Moreover, it should be noted that the differences between each adjacenttime values of the voltage measurements from the first set of voltagemeasurements (e.g., V1-V4) are equal to the differences between eachadjacent time values of the voltage measurements from the second set ofvoltage measurements (e.g., V5-V8).

At block 138, the battery control unit 36 may determine a second timeconstant τ₂ based on the four voltage measurements associated with thesecond set of voltage measurements using the same process describedabove with respect to block 134. As such, the battery control unit 36may determine the second time constant τ₂ according to Equation 8.

$\begin{matrix}{\tau_{2} = \frac{{t\; 8} - {t\; 6}}{\ln \left( \frac{{dv}\; 3}{{dv}\; 4} \right)}} & {{Equation}\mspace{14mu} 8}\end{matrix}$

At block 140, the battery control unit 36 may determine the isolationresistance (e.g., resistance 88 and/or resistance 90) based on the firstand second time constants τ₁ and τ₂. Since τ₁=R1C and τ₂=R2C, the timedifference between each adjacent voltage measurement may besubstantially equal, the battery control unit 36 may rearrange Equations4 and 5 and similar equations determined based on the second set ofvoltages to isolate C, thereby obtaining Equation 9.

$\begin{matrix}{\frac{\tau_{1}}{R\; 1} = \frac{\tau_{2}}{R\; 2}} & {{Equation}\mspace{14mu} 9}\end{matrix}$

The battery control unit 36 may isolate the capacitance C even if thevalue is known in the system because capacitor tolerances vary (e.g.,10%). Resistances R1 and R2 represent a Thevenin equivalent of theisolation resistance and the measurement circuit resistance,respectively. R1 and R2 correspond to the thevenin equivalents that theCy capacitors in the system are coupled to. By applying 2 differentknown resistances (e.g., Rcirc1 and Rcirc2) along with the unknownresistance (e.g., Riso), which would be the same in both instances, thetwo equations for R1 and R2, as provided below, will be realized. Theequations for R1 and R2 are substituted into Equation 9 to representresistances R1 and R2 as:

$\begin{matrix}{{R\; 1} = \frac{1}{\frac{1}{Riso} + \frac{1}{{Rcirc}\; 1}}} & {{Equation}\mspace{14mu} 10} \\{{R\; 2} = \frac{1}{\frac{1}{Riso} + \frac{1}{{Rcirc}\; 2}}} & {{Equation}\mspace{14mu} 11}\end{matrix}$

where R_(iso) corresponds to the isolation resistance 88 and/orresistance 90, where R_(circ1) corresponds to the equivalent resistanceof the isolation measurement circuit 12 when the first gain FET 84 isopen, and where R_(circ2) corresponds to the equivalent resistance ofthe isolation measurement circuit 12 when the first gain FET 84 isclosed, as described above with regard to the method 100 of FIG. 5. Risois the unknown resistance that the isolation measurement circuit 12 istrying to determine. Rcirc1 and Rcirc2 are the known resistance that areapplied to the system to determine Riso.

The battery control unit 36 may then synthesize Equations 10 and 11 toyield:

$\begin{matrix}{R_{final} = {\left( {{Rsa} + {Rsca}} \right)*\left( {{Rsb} + {Rscb}} \right)*\frac{{{tau}\; 2} - {{tau}\; 1}}{{{tau}\; 1*\left( {{Rsb} + {Rscb}} \right)} - {{tau}\; 2\left( {{Rsa} + {Rsca}} \right)}}}} & {{Equation}\mspace{14mu} 12}\end{matrix}$

where the Rsca and Rscb represent the different measurement circuit.Generally, Rsa and Rsca are components of Rcirc1; Rsb and Rscb arecomponents of Rcirc2. Rfinal represents the Thevenin resistance lookingout at the Cy capacitors in the system.

The battery control unit 36 may then determine the isolation resistance(e.g., resistance 88 and/or resistance 90) according to Equation 13.

$\begin{matrix}{R_{iso} = {R_{final}*\frac{R_{pul}}{R_{pul} - R_{final}}}} & {{Equation}\mspace{14mu} 13}\end{matrix}$

Rpul is a resistance pulled up to an intermediated value in the system.Rpul is provided in the case where the isolation fault is to ground. Inthis case, there would be no movement of the voltage input to theconverter since the input is already at ground with the switches off andthe application to ground. This would assure some movement of the ADCand an RC decay due to system capacitance such that the measurements maybe obtained.

By employing the isolation measurement circuit 12 to determine theisolation resistance, the battery control unit 36 is capable ofdetermining (e.g., measuring) the isolation resistance in a manner thatis not sensitive to the Cy capacitance, analog-to-digital converter(ADC) accuracy, and/or an ADC reference. Additionally, the isolationmeasurement circuit 12 may be implemented using fewer semiconductorrelay switches, is easier to calibrate, and eliminates the potentialsfor faults via the high voltage side of the battery system 12 byperforming the isolation measurement on the high voltage side, ascompared to previous systems and techniques for measuring isolationresistance that provides a voltage at a positive terminal of the batterysystem 12 and followed by a voltage at a negative terminal of thebattery system 12.

Moreover, in other embodiments, the battery control unit 36 would waituntil a waveform settled out due to the RC decay. Comparatively,implementing the above-described techniques enables the battery controlunit 36 take advantage of the RC decay and make it part of themeasurement. In this manner, the battery control unit 36 may detectfaults faster and/or earlier, thereby reducing likelihood of a faultaffecting the battery system 26, the electrical system 12, and/or thesurrounding environment.

As such, the presently disclosed embodiments use less hardware and donot involve a dedicated ADC or precision references. Further, ADCaccuracy does not affect the accuracy of the isolation resistancemeasurement. Calibration is not used to verify the isolation resistancemeasurement, and Cy capacitance does not alter the isolation resistancemeasurement. Moreover, by using the decay portion of the two RC decaywaveforms, the presently disclosed embodiments may determine theisolation resistance measurement more quickly, as compared to waitingfor each RC decay waveform to settle out to a relatively constant valuebefore performing the isolation resistance calculations.

One or more of the disclosed embodiments, alone or in combination, mayprovide one or more technical effects including monitoring the isolationresistance of the battery system 26 and disconnecting a voltage source66 (e.g., the first battery pack 28 and/or the second battery pack 30)from the battery system 26 and/or the electrical system 10. That is,when the isolation resistance is less than some threshold, a fault maybe present on in one or more isolation barrier 14. Accordingly,presently disclosed systems and techniques assist in identifying when apotential fault is present in the battery system 26 and/or theelectrical system 10 and isolates the source of the fault.

While only certain features and embodiments have been illustrated anddescribed, many modifications and changes may occur to those skilled inthe art (e.g., variations in sizes, dimensions, structures, shapes andproportions of the various elements, values of parameters (e.g.,temperatures, pressures, etc.), mounting arrangements, use of materials,colors, orientations, etc.) without materially departing from the novelteachings and advantages of the disclosed subject matter. The order orsequence of any process or method steps may be varied or re-sequencedaccording to alternative embodiments. It is, therefore, to be understoodthat the appended claims are intended to cover all such modificationsand changes as fall within the true spirit of the invention.Furthermore, in an effort to provide a concise description of theexemplary embodiments, all features of an actual implementation may nothave been described. It should be appreciated that in the development ofany such actual implementation, as in any engineering or design project,numerous implementation specific decisions may be made. Such adevelopment effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure, without undue experimentation.

1. An energy storage system configured to output at least two voltages,comprising: one or more battery packs, wherein one terminal of the oneor more battery packs is electrically coupled to a resistorrepresentative of an isolation resistance of the battery system; asemiconductor relay switch; a plurality of resistors configured toelectrically couple to the one or more battery packs via thesemiconductor relay switch; a gain field-effect transistor (FET)configured to electrically short at least one resistor of the pluralityof resistors; and a control system configured to: send a first commandto the semiconductor switch to close; acquire a first voltage waveform;send a second command to the semiconductor switch to open; send a thirdcommand to the gain FET to close; send a fourth command to thesemiconductor switch to close; acquire a second voltage waveform; anddetermine the isolation resistance based on the first voltage waveformand the second voltage waveform.
 2. The energy storage system of claim1, wherein the one or more battery packs comprise one or more lead acidbattery packs or one or more lithium ion battery packs.
 3. The energystorage system of claim 1, wherein the semiconductor relay switchcomprises light-emitting diode as an input and ametal-oxide-semiconductor field-effect transistor (MOSFET) as an output.4. The energy storage system of claim 1, wherein the semiconductor relayswitch comprises a PhotoMOS switch.
 5. The energy storage system ofclaim 1, wherein the gain FET is configured to electrically couple toground.
 6. The energy storage system of claim 1, wherein the gain FETcomprises a 200 volt rating.
 7. The energy storage system of claim 1,comprising a first switch configured to electrically couple a firstresistor to the one or more battery packs and a second switch configuredto electrically couple a second resistor to the one or more batterypacks, wherein the first resistor and the second resistor represents theisolation resistance of the battery system.
 8. The energy storage systemof claim 1, comprising one or more capacitors electrically couples to asystem ground of a vehicle and the one or more battery packs.
 9. Anon-transitory computer-readable medium comprising computer-executableinstructions configured to cause a processor to: acquire a first voltagewaveform when a plurality of resistors is electrically coupled to one ormore battery packs via a semiconductor relay switch; acquire a secondvoltage waveform when at least one resistor of the plurality ofresistors is electrically shorted via a gain field-effect transistor(FET); and determine an isolation resistance between a battery systemand a chassis based on the first voltage waveform and the second voltagewaveform.
 10. The non-transitory computer-readable medium of claim 9,wherein the processor is configured to acquire the first voltagewaveform when the gain FET is open.
 11. The non-transitorycomputer-readable medium of claim 9, wherein the processor is configuredto acquire the second voltage waveform when the gain FET is closed. 12.The non-transitory computer-readable medium of claim 9, wherein thefirst voltage waveform corresponds a resistor-capacitance (RC) decaywaveform.
 13. The non-transitory computer-readable medium of claim 9,wherein the processor is configured to determine the isolationresistance based on a first set of voltage measurements associated withthe first voltage waveform and a second set of voltage measurementsassociated with the second voltage waveform.
 14. The non-transitorycomputer-readable medium of claim 13, wherein the first set of voltagemeasurements and the second set of voltage measurements each comprise atleast four measurements.
 15. The non-transitory computer-readable mediumof claim 13, wherein each pair of adjacent voltage measurements of thefirst set of voltage measurements and the second set of voltagemeasurements comprise an equal amount of time between each other. 16.The non-transitory computer-readable medium of claim 9, wherein theisolation resistance comprises a Thevenin equivalent resistance betweenthe battery system and the chassis.
 17. An isolation measurement circuitfor measuring isolation resistance, comprising: a semiconductor relayswitch; a plurality of resistors configured to electrically couple toone or more battery packs via the semiconductor relay switch, whereinone terminal of the one or more battery packs is electrically coupled toa resistor representative of an isolation resistance between a batterysystem and a chassis; a gain field-effect transistor (FET) configured toelectrically short at least one resistor of the plurality of resistors;one or more capacitors electrically couples to a system ground of avehicle and the one or more battery packs; and a control systemconfigured to: acquire a first voltage waveform when the plurality ofresistors is electrically coupled to the one or more battery packs viathe semiconductor relay switch; acquire a second voltage waveform whenthe at least one resistor of the plurality of resistors is electricallyshorted via the gain field-effect transistor (FET); and determine theisolation resistance between the battery system and the chassis based onthe first voltage waveform and the second voltage waveform.
 18. Theisolation measurement circuit of claim 17, wherein the semiconductorrelay switch comprises light-emitting diode as an input and ametal-oxide-semiconductor field-effect transistor (MOSFET) as an output.19. The isolation measurement circuit of claim 17, wherein thesemiconductor relay switch comprises a PhotoMOS switch.
 20. Theisolation measurement circuit of claim 17, wherein the gain FETcomprises a 200 voltage amplifier device.
 21. The isolation measurementcircuit of claim 17, wherein the first voltage waveform corresponds aresistor-capacitance (RC) decay waveform.
 22. The isolation measurementcircuit of claim 21, wherein the control system is configured todetermine the isolation resistance based on a first set of voltagemeasurements associated with the first voltage waveform and a second setof voltage measurements associated with the second voltage waveform.